In a hospital setting there is always the need to monitor patient health through the evaluation of blood chemistry profile. The simplest method employed in the hospital is to use a syringe carrying a sharpened cannula at one end and insert that cannula into a vein or artery to extract a blood sample from the patient. Patients that are in the critical care units or the operating room sometimes require as many as twelve samples a day. Such frequent sampling injections potentially expose the patient to airborne bacteria and viruses which can enter the bloodstream through the opening made by the sharpened cannula.
One way to obtain a blood sample is to draw the blood from a catheter that is already inserted in the patient, either in a central venous line, such as one placed in the right atrium, or in an arterial line. Typically, existing injection sites for arterial or venous drug infusion or pressure monitoring lines are used to take periodic blood samples from the patient. Conventional mechanisms for drawing blood from the lines used for infusion or pressure monitoring utilize a plurality of stopcock mechanisms that preclude flow from the infusion fluid supply or from the pressure column drip supply, while allowing blood to flow from the patient into a collecting syringe connected to a proximal port formed in one of the stopcocks. Typically, a blunt cannula through a slit septum is used to remove the danger of sticking the nurse or clinician, in a so-called “needle-less” system.
Most early systems required a two-step operation where a first sample of fluid, generally about 5 ml in volume for intensive care environments was withdrawn into the sampling syringe and discarded. This first sample potentially included some of the infusion fluid and thus would be an unreliable blood chemistry measurement sample. After the initial sample had been discharged, the second sample was pure blood from the artery or vein and was typically re-infused to the patient.
In response to the drawbacks associated with earlier two-step sampling systems, closed systems were developed as described in U.S. Pat. No. 4,673,386 to Gordon, and more recently in U.S. Pat. No. 5,961,472 to Swendson, which are expressly incorporated by reference herein. Commercial closed systems such as the Edwards VAMP® and VAMP Plus® Venous Arterial blood Management Protection systems of Edwards Lifesciences in Irvine, Calif. feature a reservoir in the tubing line from the patient that can draw fluid past a sampling port. The clearing volume is held in the in-line reservoir, and not set-aside in a syringe for re-infusion later. The sampling systems are often used in conjunction with a pressure monitor having a transducer continually or periodically sensing pressure within the sampling line except during the draw of a blood sample.
The VAMP Plus® system conveniently utilizes a reservoir with one-handed operability, and includes a line from the patient into and out of the reservoir and to a proximal source of flushing fluid and a pressure transducer. (The standard directional nomenclature is that proximal is toward the clinician, or away from the patient, and distal is toward the patient). A stopcock valve at the reservoir controls the mode of operation. Prior to drawing a blood sample, the reservoir plunger is latched closed, though a reservoir gap allows a continuous drip of IV flushing fluid through an inlet port to an outlet port. A pressure transducer in the line proximal to the reservoir senses fluid pressure within the line and conveys the signal to a monitor. One exemplary pressure transducer used with both the VAMP® and VAMP Plus® systems is the Edwards TruWave® Disposable Pressure Transducer.
When a blood sample is to be taken, the flow of flushing or infusion fluid is halted by turning the handle of the reservoir stopcock valve. The nurse or clinician then withdraws an amount of fluid into the reservoir chamber and distal line sufficient to pull pure blood past one or more fluid sampling sites. After full retraction of the plunger, the stopcock valve closes off the reservoir from the patient and a sample of blood is taken at one or the other sampling sites. Subsequently, the clinician manipulates the stopcock valve so that the volume within the reservoir can be reinfused back into the patient by depressing the plunger, and the flushing drip and pressure monitoring resumes.
In the closed blood sampling/pressure monitoring systems described above, the pressure transducer typically includes a diaphragm exposed to the in-line fluid on one side and has a device for measuring deflection of the diaphragm on the other. Such pressure lines typically make use of relatively stiff tubing primed with a suitable physiological fluid such as saline or 5% dextrose solution as a pressure column. For adults, a bag pressurized with air surrounds the fluid supply bag to maintain a constant pressure differential in the line urging fluid toward the patient through a restrictor orifice. The slow drip of physiological fluid flushes the line to prevent clotting. Some transducers such as the TruWave® Disposable Pressure Transducer include a flush device that also can be used for sending transient pressure waves through the line. A Snap-Tab™ device of the TruWave® is a rubber tab which when pulled and then released sends a square wave through the pressure column to check the inherent frequency response of the entire system, which includes the tubing and any components attached thereto, such as the sampling ports and reservoir. Proper system frequency response is necessary for reliable blood pressure measurements. In general, a more accurate signal may be obtained with a shorter sampling line and fewer components so that the transducer is closer to the patient and there is less delay between the generation and receipt of the blood pressure signal, and less interference. However the limited amount of space available or the location of the anesthesiologist during a surgical procedure often necessitates a relatively long tubing line which degrades the signal. Furthermore, minimum functionality of the system requires various components such as sampling sites be included.
In view of the foregoing, there is a need for a blood sampling system used in conjunction with a pressure transducer that produces more accurate pressure readings.